FIG. 3 shows the structure of a conventional liquid crystal display device. In a general liquid crystal display device, an edge-light-type back light 211 is disposed on the outermost backface; and a light conductive plate 212, which emits the light of the back light toward the surface, and a light-scattering sheet 213, which makes the brightness of the light uniform, and one or plurality of light-adjusting sheets 214, having the ability to condense the light made uniform by the light-scattering sheet, in a predetermined direction, or the ability to selectively transmit or reflect a specific polarized light, are disposed, in this order, from the backface. The light passing through these films is incident to a liquid crystal cell 217, which is held between a pair of polarizing plates 215 and 216. 218 represents a cool fluorescent cathode ray tube as a light source, and 219 represents a reflection sheet. Herein, in the drawings, the same numeral represents the same member.
The anti-reflection film is usually disposed on the outermost surface of a display, which decreases the reflection of external light by using the principle of optical interference in an image display device, such as a cathode ray tube display device (CRT), a plasma display panel (PDP), and a liquid crystal display device (LCD). Namely, as an anti-reflection layer, an anti-reflection film is disposed on the display side of the polarizing plate 216 in FIG. 3.
However, in the anti-reflection film provided with only a hardcoat layer and a low-refractive-index layer on a transparent support, the low-refractive-index layer must be made to have a sufficiently lowered refractive index, to lower the reflectance. In order to decrease the average specular reflectance of, for example, an anti-reflection film using triacetyl cellulose as the support and a UV-cured coating of dipentaerythritol hexaacrylate as the hardcoat layer, to 1.6% or less, in a wavelength range between 450 nm and 650 nm, the refractive index of such a low-refractive-index layer must be 1.40 or less. Examples of materials having a refractive index of 1.40 or less include inorganic materials, such as magnesium fluoride and calcium fluoride, and organic materials, such as fluorine-containing compounds having a high fluorine content. However, these fluorine compounds lack in mechanical strength and abrasion-resistance needed for the layer disposed on the outermost surface of a display. It is therefore conventionally necessary to use a compound having a refractive index of 1.43 or more, to insure sufficient resistance to damage (abrasion).
JP-A-7-287102 (“JP-A” means unexamined published Japanese patent application) describes that the reflectance is reduced by making the refractive index of the hardcoat layer high. However, a hardcoat layer having such a high refractive index causes uneven color on the film, because of a large difference in refractive index between the hardcoat layer and a support, and the wavelength dependency of the reflectance is thereby largely fluctuated resultantly.
JP-A-7-333404 describes an anti-glare and anti-reflection film that is superior in gas barrier capability, anti-glare capability, and anti-reflection capability. However, because a silicon oxide film produced by chemical vapor deposition (CVD) is essential, the method to produce such a film is inferior in productivity to wet application.
In the meantime, the display type of LCD can roughly be classified into a birefringence mode and an optical rotation mode. A super twisted nematic liquid crystal display device utilizing the birefringence mode (referred to hereinafter as STN-LCD) employs super twisted nematic liquid crystal possessing a twisted angle exceeding 90° and steep electrooptical characteristics. Therefore, STN-LCD enables display of a large capacity due to multiplex drive. However, STN-LCD has problems such as a slow response (several hundred milliseconds) and difficulty in gradation display, and is inferior in display characteristics, as compared with those of a liquid crystal display device using active element (such as TFT-LCD and MIM-LCD).
In TFT-LCD and MIM-LCD, a twisted nematic liquid crystal possessing a twisted angle of 90° and a positive birefringence is used for displaying images. These are a display mode of TN-LCD, which is an optical rotation mode. As this mode obtains a high responsibility (several ten milliseconds) and a high contrast, this mode is advantageous in many aspects as compared with the birefringence mode. Since TN-LCD changes display colors and display contrast according to a viewing angle of looking at the liquid crystal display device (viewing angle characteristics), it involves a problem that the device is difficult in watching as compared with CRT.
JP-A-4-229828 and JP-A-4-258923 respectively disclose a proposal of providing a phase differential plate (optical compensative sheet) between a liquid crystal cell and a pair of polarizing plate for improving viewing angle characteristics. As the phase differential plate proposed in the aforesaid publications has a phase difference of almost zero (0) in the vertical direction to the liquid crystal cell, it gives no optical effect on direct front but a phase difference is realized when it is tilted. A phase difference generated in an inclined direction for the liquid crystal cell is thereby compensated. A sheet having a negative birefringence so as to compensate a positive birefringence of a nematic liquid crystal and having an inclined optic axis is effective for such optical compensative sheet.
JP-A-6-75115 and EP 0576304A1 respectively disclose an optical compensative sheet having a negative birefringence and an inclined optic axis. This sheet is manufactured by stretching a polymer such as polycarbonate or polyester, and has a main-refractive-index direction inclined to the normal line thereof. As such sheet requires an extremely complicate stretching treatment, therefore, it is extremely difficult to manufacture a uniform optical compensative sheet of a large area stably according to this method.
On the other hand, JP-A-3-9326 and JP-A-3-291601 respectively disclose a method using a liquid crystal polymer. An optical compensative sheet is thereby obtained by applying a liquid crystal polymer onto the surface of an alignment (oriented) layer on a support. However, as the liquid crystal polymer fails to show a satisfactory alignment on the alignment layer, it is impossible to enlarge the viewing angle in all directions. Further, JP-A-5-215921 discloses an optical compensative sheet (birefringent plate) that comprises a support and a liquid crystal polymeric bar-type compound having a positive birefringence. This optical compensative sheet is obtained by applying a solution of the polymeric bar-type compound onto the support and curing the compound under heating. However, the liquid crystal polymer is devoid of birefringence so that it is unable to enlarge the viewing angle in all directions.
In JP-A-8-50206, there is disclosed an optical compensative sheet characterized by a layer of a negative birefringence comprised of a compound having a discotic structure unit, wherein an angle between the discotic compound and a support is changed with respect to the direction of the depth of the layer. According to the method described therein, a viewing angle viewed from contrast is extensively enlarged in all directions and deterioration of image quality such as yellowing viewed from an incline direction is scarcely observed. With the optical compensative sheet alone, however, a deterioration in display quality based on reflection of outside light as mentioned above, cannot be improved. Thus, further improvement is required.